Asphalt is used in a diverse variety of construction applications, such as road building and roofing. The physical properties of raw asphalt, particularly at low and high temperatures, are frequently deficient in one or more aspects. Thus, a great deal of time and effort has been expended in the development of polymer modified asphalt compositions, which compositions often exhibit surprisingly improved properties.
However, it is frequently not a simple matter to prepare polymer modified asphalt. Some polymers, especially elastomers, are difficult to disperse in asphalt. Standard asphalt mixing equipment, such as a batch kettle equipped with a paddle stirrer, provides relatively low shear and is, therefore, not suited to quickly disperse elastomers in asphalt. One known solution to this problem is to grind the elastomer into a fine powder before adding it to the asphalt. This solution does provide good dispersion of the elastomer but it is relatively expensive, due to the cost associated with grinding the elastomer (i.e. powdered rubber is sold at a substantial price premium in comparison to rubber which is sold in a standard bale form).
Other attempts have been made to develop a more economical process to prepare rubber modified asphalt. U.S. Pat. No. 3,963,659 describes a process in which a rubber and asphalt are mixed in a masticator, with the asphalt being added to the masticator over an extended periof of time. As the asphalt concentration increases, the composition tends to become sticky and, therefore, the asphalt is either added in a slow continuous manner, or in small incremental charges. The process is clearly inconvenient and time consuming.
U.S. Pat. No. 4,081,502 teaches a process which overcomes some of the problems of the process of U.S. Pat. No. 3,963,659. Specifically, U.S. Pat. No. 4,081,502 teaches the use of a kneader to mix crumbs, granules or powder of polymer with asphalt. However, the mixtures are sticky and are prone to adhere to the kneader, thus requiring a time consuming cooling step as an essential element of the process. In the absence of this cooling step, substantially more than 5% of the mixture adheres to the kneader.
A fundamentally different approach to dealing with this mixing problem is suggested by U.S. Pat. No. 4,490,493, namely, the selection of compatible asphalt-polymer systems. However, the teachings of this patent are not particularly helpful to a person who would like to incorporate ethylene-propylene polymers into asphalt.
In summary, a person who is interested in directly incorporating raw ethylene-propylene rubber into asphalt is generally faced with a difficult dispersion problem. Attempts have been made to improve the dispersion of rubber into asphalt by developing processes to premix some of the asphalt with rubber, but such processes have not been entirely satisfactory. Thus, the processes of the prior art are encumbered by one or more expensive or inconvenient steps.